There are many circumstances in which it is desirable or necessary to be able to transmit information in documentary form in a secure manner such that the information can be seen only by the intended recipient. The need for such security arises not only from a social and personal perspective, for example in the transmission of payment details on payslips and the like, but also for more serious reasons for the prevention of criminal activity in the transmission of personal information numbers (PIN numbers) associated with credit cards and other such information.
A primary consideration in the production of documents bearing such information is that the information must be applied to the document in such a way that no other person, not even the person operating the machinery by which the information is applied to the document, can gain access to it.
Prior art attempts to produce satisfactory such documents have included preliminarily closed envelopes bearing, on their inside faces, micro-encapsulated ink particles which, when struck by an impact instrument, rupture and release the ink. In order to apply the information to the document it is, however, necessary to have a so-called impact printer (dot-matrix, daisy wheel or the like) by which the individual letters or numerals of the information can be impressed onto the document in order to cause rupture of the micro-encapsulated ink particles. In order to prevent the information inside the envelope from being viewed from the outside, one face of the document is provided with obscuration printing in the form of swirls and curlicues in an irregular pattern such as to provide a background against which the printed information cannot be distinguished.
However, the majority of establishments currently utilise printing equipment other than impact printers, for example laser printers and bubble jet printers which cannot utilise microencapsulated ink materials since they need an impact to rupture them. One attempt at producing a document for secure information which can be printed utilising a laser, ink jet or bubble jet printer comprises a single sheet of material which, after printing, can be folded and secured by adhesive at the periphery to enclose the sensitive information within it in the manner of an envelope. However, this has the disadvantage that it requires a folding machine. Such machines are expensive and unreliable and require constant supervision to ensure that the folding operation is performed properly.